<template>
	<ul class="list">
		<li class="item" 
			v-for="item of letters" 
			:key="item" 
			:ref="item" 
			@click="handleLetterClick"
			@touchstart="handleTouchStart" 
			@touchmove="handleTouchMove" 
			@touchend="handleTouchEnd"
		>
			{{item}}
		</li>
	</ul>
</template>

<script>
export default {
	name:'CityAlphabet',
	props:{
		cities:Object
	},
	computed:{
		letters(){
			const letters = []
			for(let i in this.cities){
				letters.push(i)
			}
			return letters
		}
	},
	data () {
		return{
			touchStatus:false,
			startY:0,
			timer:null
		}
	},
	updated() {
		this.startY = this.$refs['A'][0].offsetTop
	},
	methods:{
		//事件对象e
		handleLetterClick(e){
			this.$emit('change',e.target.innerText)
		},
		handleTouchStart(){
			this.touchStatus = true
		},
		handleTouchMove(e){
			if(this.touchStatus == true){
				//节流控制
				if (this.timer) {
					clearTimeout(this.timer)
				}
				this.timer = setTimeout(() => {
					const touchY = e.touches[0].clientY - 79
					const index = Math.floor((touchY - this.startY) / 20)
					if(index >= 0 && index < this.letters.length){
						this.$emit('change',this.letters[index])
					}
				},16)
			}
		},
		handleTouchEnd(){
			this.touchStatus = false
		}
	}
}
</script>

<style lang="stylus" scoped>
@import '~styles/varibles.styl'
.list
	/*垂直方向居中*/
	display:flex
	flex-direction:column
	justify-content:center
	/*垂直方向居中结束*/
	position:absolute
	top:1.58rem
	right:0
	bottom:0
	width:.4rem
	.item
		text-align:center
		line-height:.4rem
		color:$bgColor
</style>
